Abstract [en]

Organic semiconductors usually demonstrate crystal structure dependent electronic properties, and through precise control of film structure, the performance of novel organic electronic devices can be greatly improved. Understanding the crystal structure dependent charge-transfer mechanism thus becomes critical. In this work, we have prepared amorphous titanyl phthalocyanine films by vacuum molecular beam evaporation and have further crystallized them through vacuum annealing. In the crystalline phase, an excited electron is rapidly transferred into neighboring molecules; while in the amorphous phase, it is mainly localized and recombines with the core hole as revealed by resonant photoemission spectroscopy (RPES). The fast electron transfer time is determined to be around 16 fs in the crystalline film, which is in good agreement with the charge-transfer hopping time estimated from the best device performance reported. The crystallized film shows more p-type characteristics than the amorphous with all the energy levels shifting toward the vacuum level. However, the greatly improved charge transfer is assigned to the molecular orbital coupling rather than this shift. From density functional theory and RPES, we specify the contribution of two differently coordinated nitrogen atoms (N2c and N3c) to the experimental results and illustrate that the N3c related orbital has experienced a dramatic change, which is keenly related to the improved charge transfer.

Abstract [en]

The photosensitive molecule adsorption on titanium dioxide (TiO2) forms the so-called “dye sensitized TiO2” system, a typical organic/oxide heterojunction, which is of great interest in catalysis and energy applications, e.g. dye-sensitized solar cell (DSSC). Traditionally, the transition metal complex dyes are the focus of the study. However, as the fast development of the organic semiconductors and invention of new pure organic dyes, it is necessary to expand the research horizon to cover these molecules and concrete the fundamental understanding of their basic properties, especially during sensitization.In this work, we focus on two different photosensitive molecules: phthalocyanines and triphenylamine-based dyes. Phthalocyanines are organic semiconductors with symmetric macro aromatic molecular structures. They possess good photoelectrical properties and good thermal and chemical stability, which make them widely used in the organic electronic industries. Triphenylamine-based dyes are new types of pure organic dyes which deliver high efficiency and reduce the cost of DSSC. They can be nominated as one of the strong candidates to substitute the ruthenium complex dyes in DSSC. The researches were carried out using classic surface science techniques on single crystal substrates and under ultrahigh vacuum condition. The photosensitive molecules were deposited by organic molecular beam deposition. The substrate reconstruction and ordering were checked by low energy electron diffraction. The molecular electronic, geometric structures and charge transfer properties were characterized by photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and resonant photoelectron spectroscopy (RPES). Scanning tunneling microscopy is used to directly image the molecular adsorption.For phthalocyanines, we select MgPc, ZnPc, FePc and TiOPc, which showed a general charge transfer from molecule to the substrate when adsorbed on rutile TiO2(110) surface with 1×1 and 1×2 reconstructions. This charge transfer can be prevented by modifying the TiO2 surface with pyridine derivatives (4-tert-butyl pyridine (4TBP), 2,2’-bipyridine and 4,4’-bipyridine), and furthermore the energy level alignment at the interface is modified by the surface dipole established by the pyridine molecules. Annealing also plays an important role to control the molecular structure and change the electronic structure together with the charge transfer properties, shown by TiOPc film. Special discussions were done for 4TBP for its ability to shift the substrate band bending by healing the oxygen vacancies, which makes it an important additive in the DSSC electrolyte. For the triphenylamine-based dye (TPAC), the systematic deposition enables the characterization of the coverage dependent changes of molecular electronic and geometric structures. The light polarization dependent charge transfer was revealed by RPES. Furthermore, the iodine doped TPAC on TiO2 were investigated to mimic the electrolyte/dye/TiO2 interface in the real DSSC.The whole work of this thesis aims to provide fundamental understanding of the interaction between photosensitive molecules on TiO2 surfaces at molecular level in the monolayer region, e.g. the formation of interfacial states and the coverage dependent atomic and electronic structures, etc. We explored the potential of the application of new dyes and modified of the existing system by identifying their advantage and disadvantage. The results may benefit the fields of dye syntheses, catalysis researches and designs of organic photovoltaic devices.